Microvaristor filled polymers show non-linear current-voltage characteristics and can be used for over-voltage protection purposes, for example to protect sensitive electronics from electrostatic discharges. Nonlinear materials composed of a polymer matrix filled with conductive and/or semi-conductive and/or insulating particles are known and used for over-stress protection of electronic chips. The protection voltage level needed for electronics is low, which means that the material should have either a low clamping or switching voltage or should be very thin.
EP 0 992 042 (WO 99/56290) discloses varistor composites comprising microvaristor filler particles embedded in a matrix and a production method for such varistor composites. The non-linear filler material comprises sintered microvaristor granulate made of doped zinc oxide. The switching voltage of the composite can be reduced by decorating the microvaristor particles with micro-sized metallic flakes. In the decoration process, in a first step the microvaristor particles and the metallic flakes are intimately mixed, and in a second step the flakes are bonded to the microvaristor particles by heat treatment. This process suffers from the fact that micrometer metal particles tend to agglomerate. Breaking of the agglomerates in a dry mill is not possible, because the metal is ductile. Instead, the agglomerates tend to solidify by cold welding. Therefore the quality of the decoration strongly depends on the handling of the metallic powder, leading to non-reproducible non-linear properties of the compounds.
In the article by F. Greuter et al., “Microvaristors: Functional Fillers for Novel Electroceramic Composites”, J. Electroceramics, 13, 739-744 (2004), varistor composites containing ZnO microvaristors embedded in a polymer matrix are disclosed for electrostratic discharge (ESD) protection of electronics. The ZnO microvaristor particles show strong nonlinearities of their electrical resistance as a function of the applied electric field. The nonlinear behaviour of the composite material depends on the microvaristor particle nonlinearities, their packing arrangement and the microscopic properties of the particle-particle contacts. By decorating the microvaristors with small metal flakes, the switching field of the composite is reduced and the energy absorption is improved. The conventional decoration process using metallic flakes suffers from the agglomeration problems as discussed above. For applications in ESD protection, polymers filled with decorated microvaristor particles can be molded, casted, etc. onto the electronic elements to be protected.